Jeff Mitton: Crab spiders change colors to match their plants

Blue lupines and wild bergamot were blooming near the top of Flagstaff Mountain, attracting an assortment of butterflies.

I was enjoying the fluttering when I noticed a northwestern fritillary, Speyeria hesperis, hanging upside down and motionless from a bergamot. A goldenrod crab spider, Misumena vatia, had its fangs embedded in the butterfly's body and was sucking out its fluids.

This spider was quite a hunter, for it had killed an insect bigger than itself. Although crab spiders spin silk for drop lines, they do not build webs to catch prey. Instead, they find a spot where they are well camouflaged and lie motionless, waiting for an insect to approach. When an insect gets close, they pounce, grab and inject venom.

Spiders do not have jaws like vertebrates, but manage with two pairs of mouthparts. Fangs impale the prey and inject venom. Pedipalps, which look like a pair of small legs, are used to manipulate tiny prey items and macerate and liquefy tissue. Spiders inject enzymes to dissolve tissue and then suck out the fluids.

Crab spiders are commonly on flowering plants, for they eat the insect herbivores and pollinators attracted by the plant. I have found them on blanketflower, wild bergamot, lupines, tarweed, dandelions and goldenrod.

Goldenrod crab spiders do not always look the same, but vary with their background. For example, some are all yellow with red slashes on each side of the abdomen, or cream or white with red slashes. Some have a white abdomen with red slashes, but all other parts of the body green. Sometimes the slashes are brilliant red, sometimes faded and barely discernible.

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Among the 3,000 species of crab spiders around the world, the goldenrod crab spider is unique in its ability to change its color to match its background. Experimental studies have demonstrated that M. vatia is aware of the color of its background, for if it was moved to a plant where it was conspicuous, it immediately moved to a plant where it was camouflaged.

Furthermore, if a yellow spider is conspicuous on its substrate and unable to move, it will resorb the yellow pigments and sequester them, or even excrete them. As the yellow pigments diminish, the crab spider turns to cream or white. If a white spider feels the need to change from cream to yellow, it can mobilize sequestered pigments or synthesize new pigments. It takes about three days to change color.

Do plants benefit by hosting spiders? Crab spiders hunt from plants, lurking on leaves or even hiding in flowers, waiting to attack insect herbivores and pollinators. The host plants might benefit if it is protected from herbivores, but might be disadvantaged if the spider eats too many pollinators.

Experimental studies showed that plants with resident crab spiders had fewer herbivores chewing on them -- that was certainly a benefit. Plants with no damage to their flower heads produced similar numbers of seeds whether they had spiders or not. However, plants with damaged seed heads produced more seeds if they hosted spiders. So crab spiders reduce the numbers of herbivores and in some circumstances actually increase the production of seeds.

Goldenrod crab spiders practice cannibalism, but they are fussy about whom they eat. For example, they do not eat offspring. But females may turn on a male after mating.

The probability of cannibalism increases with the age of the male. Perhaps older males have lost speed and agility and are easy prey.

I wonder if elderly males on the brink of cannibalism take solace in the knowledge that they will be directly contributing to the nutrition of their offspring. While I can't say for sure, I doubt that they would be afforded the time to be introspective and philosophical.

Jeff Mitton (mitton@colorado.edu) is a professor in the Department of Ecology and Evolutionary Biology at the University of Colorado.

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